Hit Your Target Gravity

When your neighborhood pub stops serving your favorite English bitter, what can you do? Make your own, of course. Homebrewers thrive on the challenge of recreating the round malts and bracing hops of popular beer styles. Armed with a hydrometer, experienced homebrewers can construct a beer from the ground up, starting with specific gravity.

A hydrometer measures the density of a liquid solution by comparing the weight of that solution with the weight of pure water, measured as 1.000 at 60° F (16° C). Expressed as specific gravity, this density represents the amount of soluble matter, such as sucrose, dissolved in water. For example the density of a five-gallon extract brew using seven pounds of dry malt extract measures 1.055 specific gravity after boiling for an hour, cooling, and mixing well with pre-boiled water.

The first gravity reading measures original gravity (OG), which includes the percentage of soluble sugars available to the yeast to metabolize into alcohol and carbon dioxide. Original gravity is one half of the equation that determines alcohol percentage. The other half is final gravity (FG), sometimes called terminal gravity. This is the measurement taken after the yeast is finished consuming fermentable sugars, leaving behind non-fermentable sugars, proteins, and peptides.

To successfully replicate popular beer styles homebrewers must produce a soluble sugar solution with an original gravity measured within that style’s specific range. Or, actually, generate the proper circumstances to produce the solution.

The fermentation cycle must then produce a final gravity within a specific range, producing the correct alcohol percentage, body, and balance for the style.

The Empirical MethodWhile specific gravity sounds very scientific, brewers have only recently made brewing decisions based on brewing science and theory. Traditionally, brewers relied on trial and error. They depended on keen observations to understand how process changes affected the final product. Homebrewers often rely on this empirical method to control their brews, including adjusting them to obtain specific original and final gravity levels.

All brewers should maintain strict notations of each brew to observe the effects of procedure and ingredient deviations from batch to batch. This information can be used to make critical changes in future brews.

Notations should feature as many observable and measurable elements of the brewing process as possible, from steeping grains to final taste. Include the exact ingredients of the batch recipe, amount of time wort is boiled, when hops are added, wort temperature when cool, original gravity, length of time wort is aerated, length of time before yeast begins to work, fermentation temperature, amount of time from when the yeast was pitched to the end of noticeable fermentation, final gravity, and final taste. All notations should be dated.

Another important element of batch notation should be labeled simply Batch Notes. List unusual observations, including problems and possible solutions, which may lead to necessary changes in the brewing schedule.

Extract brewers should also include the amount of pre-boiled, cold water added to the primary fermenter to reach the desired measure, and name and amount of specialty grains used.

All-grain brewers need to include more comprehensive notations: mash-in temperature(s) and time(s), strike temperature of water, amount of time wort is recirculated through lauter tun, temperature of sparge water, and length of time mash is sparged to reach desired measure.

Original Gravity/ExtractsWhen building a beer, brewers must account for color and malt flavor by using the correct combination of specialty grains or malt extract. These combinations act in conjunction with the amount of pale malt or total extract to determine the original gravity.

For beginning brewers original gravity may be one of the more frustrating targets to reach and then consistently maintain brew after brew. What can be more disconcerting than brewing a perfect English special bitter at 1.042 OG then, after using the same ingredients and procedures, having the next batch weigh in at 1.030?

Extract brewers may find that 6.5 pounds of the same brand of extract used just last month generated a completely different original gravity when brewed again under similar conditions. Many extract sources maintain a variable range of malt extract per pound, making accurate predictions of original gravity difficult. Homebrewers combat this problem by blending dry and syrup extract, with one or the other known for consistent malt content. In all instances refer to past batch notations to fine-tune a recipe.

For example one recipe for five gallons of American pale ale called for 6.5 pounds of a popular brand of unhopped light dry extract. The original gravity measured 1.045 in one batch but 1.058 in the next brew using the same product. Another recipe for the same style ale called for 6.6 pounds of a popular brand of unhopped light syrup extract that measured 1.052 OG for two
subsequent batches.

Dividing the last two numbers of the original gravity measurement by the weight in pounds of extract used produces the extract potential per pound for each extract source:

Combining the syrup and dry extracts may stabilize the original gravity while maintaining a taste appropriate for style. The recipe now calls for three pounds of light dry extract and 3.3 pounds of light syrup extract. Reversing the above equation can help estimate the original gravity:

The improved American pale ale recipe will now produce a brew between 1.047 (OG) and 1.053 (OG), both within the range for the style, with the malt now perfectly balanced. Only experimentation and strict notation will help turn any problematic recipe into a fine beer.

Original GravityAll-grain brewers may encounter the same problems with inconsistent original gravities from brew to brew. They must then examine the crush of the grain, the temperature of the mash schedule, amount of time the mash is held consistently within a range of 150° F to 158° F, and sparging techniques.

Mashing begins and ends with extraction efficiency by drawing out as much soluble substance of the malt as possible. At first an iodine test can help reveal whether any problems exist. When you’ve reached a somewhat consistent level of mashing efficiency, you can replicate any beer style you want, hitting those target original gravities with each brew.

As with extract brews, malted barley should maintain a consistent level of extract potential per pound. For example batch notes describe how an American pale ale recipe that called for 11 pounds of two-row pale malt and eight ounces of crystal malt measured at 1.052 OG for a brew and then measured 1.042 OG for the next batch using the same grains and brewing schedule. The notes state that every observable element, up to the evaporation rate during the boil, was the same for both brews.

Dividing the last two numbers of the original gravity by the weight of the malt in pounds used produces the extract potential per pound:

Batch one had 1.042 OG using 11.5 pounds of grain: 42 divided by 11.5 equals 3.65 (specific gravity points per pound of grain).

Batch two had 1.052 OG using 11.5 pounds of grain: 52 divided by 11.5 equals 4.5 (specific gravity points per pound of grain).

With such a disparity in original gravities and with the batch notes suggesting that nothing in the brewing schedule could cause such a different result, the problem must exist before mashing.

Batch one suggests a serious challenge because 3.65 specific gravity points per pound is very low. Either the crush of the grain may be inconsistent from brew to brew, with too many whole grains comprising the total grist, or the grain may be old and stale or stored incorrectly, with too much moisture interfering with the stability of the malt. Again, strict notation and observation will usually point to an area that needs attention when problems arise.
Final Gravity
Final gravity measures the attenuation of the beer, which is the reduction of the wort’s density caused by the fermentation of sugars into alcohol and carbon dioxide. Fermentation also leaves behind dextrins (non-fermentable sugars), proteins, and peptides, all of which combine to form the density of final gravity readings. These dextrins, proteins, and peptides also provide body and mouthfeel.

Final gravity readings help define each beer because the density of the finished brew determines alcohol content and overall balance. An American pale ale with an original gravity of 1.054 but a final gravity of 1.020 missed the mark because the final density of the beer will provide too much body, which then masks the hop bitterness, leaving a beer malty rich and out of balance for the style.

Brewers should always examine all possible causes of high final-gravity readings, such as improper aeration of the wort, low fermentation temperature, lack of viable yeast, or too many dextrins. Low final-gravity readings can be caused by wild yeast contamination, bacterial contamination, or not enough dextrins. A careful examination of sanitation, brewing procedures, and yeast source can solve many final-gravity problems.

Extract brewers may also find that the extract source contained too few dextrins for a high final gravity or too many dextrins for a low final gravity. As when fine-tuning original gravity, combining extract syrup with dry extract may solve this problem as well. For extract brewers adjusting the amount of available dextrins is the only way to adjust final gravity and mouthfeel in a successful brew, because malt extract does not include the proteins and peptides generated by all-grain mashing. Only experimentation and strict notation will help extract brewers obtain target final gravities.

All-grain brewers should examine the mash schedule. Well-modified malt mashed in with a single-step infusion method should stabilize within a range of 150° to 158° F. A mash held at 150° F will produce mostly fermentable sugars, generating a delicate brew with light body and mouthfeel. A mash held at 158° F will produce a blend of fermentable and non-fermentable sugars, creating a brew with either medium or full body.

For example a Continental style stout with a 1.055 OG finished at 1.008 FG. Batch notes state that the mash temperature stabilized at 158° F for an hour but actually started at 148° F, requiring 20 minutes of forced heat to raise the temperature. Well-modified malt held for 15 minutes within the 150° to 158° F range will reach total saccharification, with all starches converted into sugars. The stout seemed thin for style. In the short time that heat was applied to raise the temperature, the starch converted primarily to fermentable sugars, well before reaching the higher range of temperature required to produce dextrins. Batch notes then suggest that the strike temperature of the water added to the mash to reach conversion temperatures should be raised so that the mash will immediately reach 158° F before conversion.

Another solution includes the addition of cara-malts, including cara-pils, cara-Vienne and cara-Munich, into the total grist bill. These specialty grains are designed to produce dextrins. Eight ounces of any of the above cara-malt will raise the final gravity of a five-gallon brew.

All-grain brewers enjoy the ability to lightly raise or lower mash temperatures while adding or decreasing, if necessary, dextrin malts to produce a perfect blend of fermentable sugars, dextrins, proteins, and peptides. Only experimentation and strict notation will help all-grain brewers obtain target final gravities in each batch.